Harden the native stack dumping decision.

We've seen monkey crashes in this code, though I haven't been
able to reproduce them in my own SIGQUIT stress tests. Address
the two most likely causes of trouble: dumping the signal catcher's
own thread (which will always be runnable), and assuming that the
Method* pulled from the save area is non-NULL.

(cherry-pick of feddac5b7718dd8141391bfeb6359f1906542823.)

Bug: 8596028
Change-Id: I7a70ce047c8285715eb7bbb9438e8ef5d81fc59c
1 file changed